internal/envoy: always extract envoy

[travisgroth]

Summary

We currently optimize for startup time and attempt to cache the envoy binary once we've extracted it on a system. This creates starting security conditions that are hard to validate even with checksumming of the binary and atomic creation.

Changes:

• switch to a randomized subdirectory created on-demand, to help ensure we are running the same binary we checksum
• clean the entire base directory before extraction, so that we don't consume additional disk space every time we start on a system
• always extract a fresh copy of envoy

Related issues

#1908

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[travisgroth]

Just in case the operation takes time, let people know why we're "paused".

[codecov]

Codecov Report

Merging #2160 (3489d57) into master (699ebf0) will decrease coverage by 0.1%.
The diff coverage is 0.0%.

@@           Coverage Diff            @@
##           master   #2160     +/-   ##
========================================
- Coverage    59.3%   59.1%   -0.2%     
========================================
  Files         173     173             
  Lines       11670   11670             
========================================
- Hits         6925    6904     -21     
- Misses       3950    3966     +16     
- Partials      795     800      +5     

Impacted Files	Coverage Δ
internal/envoy/embed.go	0.0% <0.0%> (ø)
internal/envoy/envoy.go	3.0% <0.0%> (ø)
internal/controlplane/events.go	64.5% <0.0%> (-13.0%)	⬇️
pkg/storage/inmemory/stream.go	69.3% <0.0%> (-4.1%)	⬇️
pkg/grpc/databroker/syncer.go	93.7% <0.0%> (-2.5%)	⬇️
pkg/storage/redis/redis.go	68.1% <0.0%> (-2.5%)	⬇️
pkg/storage/inmemory/backend.go	83.2% <0.0%> (-2.3%)	⬇️
internal/controlplane/xdsmgr/xdsmgr.go	70.8% <0.0%> (-1.6%)	⬇️
authenticate/state.go	65.4% <0.0%> (-0.9%)	⬇️
internal/sessions/cookie/cookie_store.go	97.6% <0.0%> (+<0.1%)	⬆️
... and 1 more